The present invention relates to a semiconductor dynamic quantity sensor such as an acceleration sensor, which includes a movable electrode that can move along two axes and a fixed electrode that partially faces the movable electrode across a gap for detection. The dynamic quantity related to a force applied to the sensor along the two axes is detected on the basis of the changes in capacitance between the electrodes.
An acceleration sensor disclosed in JP-A-5-249138 is an example of the semiconductor dynamic quantity sensors, or two-axis sensors, capable of detecting acceleration along two orthogonal axes. A semiconductor sensor in FIG. 8 is proposed by the inventor as an acceleration sensor of that kind. The sensor in FIG. 8 can be manufactured using a well-known semiconductor manufacturing process. In the manufacturing process, grooves are formed in a semiconductor substrate 12 to form a movable electrode 30 and fixed electrodes J40, J50, J60, J70.
In the semiconductor sensor in FIG. 8, the movable electrode 30 can move along a first direction X and a second direction Y, which are orthogonal to each other on a surface parallel to the semiconductor substrate 12, in response to the acceleration of the sensor. As shown in FIG. 8, the movable electrode 30 is movably supported by beams 33, 34, so movable electrode 30 moves along the first direction X with the spring-like action of the beam 33 when there is acceleration along the first direction X, and along the second direction Y with the spring-like action of the beam 34 when there is acceleration along the second direction Y, which is orthogonal to the first direction X.
As shown in FIG. 8, movable electrode portions 36, which are shaped like a comb with teeth and include a plurality of movable electrode teeth, are located at two ends of the movable electrode 30 along the first direction X and at two ends of the movable electrode 30 along the second direction Y. Each of the fixed electrodes J40, J50, J60, J70 is shaped like a comb with teeth and includes a plurality of fixed electrode teeth. Each of the fixed electrodes J40, J50, J60, J70 is supported and fixed in the semiconductor substrate 12.
Each of the fixed electrodes J40, J50, J60, J70 interleave with each movable electrode portion 36 such that each fixed electrode tooth of the fixed electrodes J40, J50, J60, J70 faces each movable electrode tooth of each movable electrode portion 36 at the two ends along the first direction X and at the two ends along the second direction Y of the movable electrode 30, respectively. The fixed electrodes J40, J50, which interleave with the movable electrode portions 36 at the two ends of the movable electrode 30 along the second direction Y, are a pair of first fixed electrodes J40, J50.
Two first detection capacitances CX1, CX2 are formed with the first fixed electrodes J40, J50 and the movable electrode portions 36. As shown in FIG. 8, one of the first detection capacitance CX1 is formed between the fixed electrode J40 and one of the movable electrode portions 36, and the other of the first detection capacitance CX2 is formed between the fixed electrode J50 and another one of the movable electrode portions 36.
The fixed electrodes J60, J70, which interleave with the movable electrode portions 36 at the two ends of the movable electrode 30 along the first direction X, are a pair of second fixed electrodes J60, J70. Two second detection capacitances CY1, CY2 are formed with the second fixed electrodes J60, J70 and the movable electrode portions 36. As shown in FIG. 8, one of the second detection capacitance CY1 is formed between the fixed electrode J60 and one of the movable electrode portions 36, and the other of the second detection capacitance CY2 is formed between the fixed electrode J70 and another one of the movable electrode portions 36. In FIG. 8, each capacitance CX1, CX2, CY1, and CY2 is represented by a capacitor symbol.
As shown in FIG. 8, pads P30, P40, P50, P60, P70, which are made of aluminum and so on and respectively correspond to the movable electrode 30 and the fixed electrodes J40, J50, J60, J70, are located on the semiconductor substrate 12. Each electrode 30, J40, J50, J60, J70 is electrically connected to each corresponding pad P30, P40, P50, P60, P70. Although not illustrated, each pad P30, P40, P50, P60, P70 is electrically connected to an outside circuit or a wiring line by, for example, wire bonding.
When the movable electrode 30 moves along the first direction X under acceleration in the two axes sensor of FIG. 8, the acceleration is detected on the basis of changes in the first detection capacitances CX1, CX2. When the movable electrode 30 moves along the second direction Y under acceleration, the acceleration is detected on the basis of changes in the second detection capacitances CY1, CY2.
Specifically, as shown in FIG. 9, the change in detection capacitances CX1, CX2, CY1, CY2 is detected using a switched capacitor circuit 200 in an external circuit, to which the semiconductor sensor in FIG. 8 is electrically connected. The switched capacitor circuit 200 is a C-V converter circuit and includes a capacitor 210, which has a capacitance Cf, a switch 220, and a differential amplifier circuit 230 for converting an inputted capacitance into a voltage as the output of the switched capacitor circuit 200.
As shown in FIG. 9, a pair of carrier waves Vcc are sent to the first fixed electrodes J40, J50 such that the electrodes J40, J50 have a phase opposite to each other. At the same time, another pair of carrier waves Vcc are sent to the second fixed electrodes J60, J70 such that the electrodes J60, J70 have a phase opposite to each other. The pairs of carrier waves Vcc are sent from the external circuit through each of the pads P30, P40, P50, P60, P70. In the mean time, the switch 220 in the switched capacitor circuit 200 is turned on and off with a predetermined timing.
In that case, the acceleration is detected as an output S′ shown in eq. 1.S′=[(CY2−CY1)+(CX2−CX1)]×Vcc/Cf  (eq. 1)
That is, in the proposed two-axis sensor in FIG. 8, the acceleration is detected on the basis of the sum of the differential output (CX2−CX1) from the first detection capacitances CX1, CX2 and the differential output (CY2−CY1) from the second direction capacitance CY1, CY2, when the movable electrode 30 moves along the first direction X or the second direction Y under the acceleration.
Specifically, in the proposed two-axis sensor in FIG. 8, when the movable electrode 30 moves along the first direction X toward, for example, the right hand side in FIG. 8, the distance between each fixed electrode tooth of one of the first fixed electrodes J40 and each movable electrode tooth of corresponding movable electrode portion 36 becomes wider, while the distance between each fixed electrode tooth of the other first fixed electrode J50 and each movable electrode tooth of corresponding movable electrode portion 36 becomes narrower. As a result, the acceleration along the first direction X is detected on the basis of the differential output (CX2−CX1) from the first detection capacitances CX1, CX2.
On the other hand, when the movable electrode 30 moves, for example, upward in FIG. 8 along the second direction Y, the distance between each fixed electrode tooth of one of the second fixed electrodes J60 and each movable electrode tooth of corresponding movable electrode portion 36 becomes wider, while the distance between each fixed electrode tooth of the other second fixed electrode J70 and each movable electrode tooth of corresponding movable electrode portion 36 becomes narrower. As a result, the acceleration along the second direction Y is detected on the basis of the differential output (CY2−CY1) from the second direction capacitances CY1, CY2.
However, when the movable electrode 30 moves along the first direction X, not only the first detection capacitances CX1, CX2 change, but also the second detection capacitances CY1, CY2 slightly change because the sizes of the overlapping areas, in which each fixed electrode tooth of the second fixed electrode J60, J70 and each movable electrode tooth of corresponding movable electrode portion 36 face each other, also changes.
When the movable electrode 30 moves along the first direction X toward, for example, the right hand side in FIG. 8, the overlapping area between each fixed electrode tooth of one of the second fixed electrodes J60 and each movable electrode tooth of corresponding movable electrode portion 36 becomes larger, while the overlapping area between each fixed electrode tooth of the other second fixed electrode J70 and each movable electrode tooth of corresponding movable electrode portion 36 becomes smaller. In other words, in addition to the expected output for the acceleration detection in the first direction X, which is the differential output (CX2−CX1) from the first detection capacitance CX1, CX2, a change in the differential output (CY2−CY1) from the second detection capacitances CY1, CY2, which are used to detect the acceleration detection in the second direction Y, is also picked up as noises.
When the movable electrode 30 moves, for example, upward in FIG. 8 along the second direction Y, the overlapping area between each fixed electrode tooth of one of the first fixed electrodes J40 and each movable electrode tooth of corresponding movable electrode portion 36 becomes smaller, while the overlapping area between each fixed electrode tooth of the other first fixed electrode J50 and each movable electrode tooth of corresponding movable electrode portion 36 becomes larger. In other words, in addition to the expected output for the acceleration detection in the second direction Y, which is the differential output (CY2−CY1) from the second detection capacitance CY1, CY2, a change in the differential output (CX2−CX1) from the first detection capacitances CX1, CX2, which are used to detect the acceleration detection in the first direction X, is also picked up as noises.
The above phenomena will be described using mathematical equations. Provided that each of the first and second detection capacitances CX1, CX2, CY1, CY2 is initially C0, the changes in the first detection capacitances CX1, CX2 is respectively ΔCx, and the changes in the second detection capacitances CY1, CY2 is respectively ΔCy. When the proposed sensor in FIG. 8 is not under acceleration,CX1=CX2=CY1=CY2=C0.
When the movable electrode 30 moves along the first direction X toward the right hand side in FIG. 8, one of the first detection capacitances CX1 decreases because the distance between each fixed electrode tooth of one of the first fixed electrodes J40 and each movable electrode tooth of corresponding movable electrode portion 36 becomes wider, while the other first detection capacitance CX2 increases because the distance between each fixed electrode tooth of the other first fixed electrode J50 and each movable electrode tooth of corresponding movable electrode portion 36 becomes narrower.
At the same time, one of the second detection capacitances CY1 increases because the overlapping area between each fixed electrode tooth of one of the second fixed electrodes J60 and each movable electrode tooth of corresponding movable electrode portion 36 becomes larger, while the other second detection capacitance CY2 decreases because the overlapping area between each fixed electrode tooth of the other second fixed electrode J70 and each movable electrode tooth of corresponding movable electrode portion 36 becomes smaller. In that case, the output S′ is expressed in eq. 2:                               S          ′                =                                            {                                                (                                      CY2                    -                    CY1                                    )                                +                                  (                                      CX2                    -                    CX1                                    )                                            }                        ×                          Vcc              /              Cf                                =                                                    [                                                      {                                                                  (                                                  C0                          -                                                      Δ                            ⁢                                                                                                                  ⁢                            Cy                                                                          )                                            -                                              (                                                  C0                          +                                                      Δ                            ⁢                                                                                                                  ⁢                            Cy                                                                          )                                                              }                                    +                                      {                                                                  (                                                  C0                          +                                                      Δ                            ⁢                                                                                                                  ⁢                            Cx                                                                          )                                            -                                              (                                                  C0                          -                                                      Δ                            ⁢                                                                                                                  ⁢                            Cx                                                                          )                                                              }                                                  ]                            ×                              Vcc                /                Cf                                      =                                                            {                                                            (                                              C0                        -                                                  Δ                          ⁢                                                                                                          ⁢                          Cy                                                -                        C0                        -                                                  Δ                          ⁢                                                                                                          ⁢                          Cy                                                                    )                                        +                                          (                                              C0                        +                                                  Δ                          ⁢                                                                                                          ⁢                          Cx                                                -                        C0                        +                                                  Δ                          ⁢                                                                                                          ⁢                          Cx                                                                    )                                                        }                                ×                                  Vcc                  /                  Cf                                            =                              2                ×                                  (                                                            Δ                      ⁢                                                                                          ⁢                      Cx                                        -                                          Δ                      ⁢                                                                                          ⁢                      Cy                                                        )                                ×                                  Vcc                  /                  Cf                                                                                        (                  eq          .                                          ⁢          2                )            
As shown in eq. 2, when detecting acceleration along the first direction X, in addition to the expected output 2ΔCx, the differential output −2ΔCy from the second detection capacitances CY1, CY2, which are used to detect the acceleration detection in the second direction Y, is also picked up as noises.
When the movable electrode 30 moves along the second direction Y upward in FIG. 8, one of the second detection capacitances CY1 decreases because the distance between each fixed electrode tooth of one of the second fixed electrodes J60 and each movable electrode tooth of corresponding movable electrode portion 36 becomes wider, while the other second detection capacitance CY2 increases because the distance between each fixed electrode tooth of the other second fixed electrode J70 and each movable electrode tooth of corresponding movable electrode portion 36 becomes narrower. At the same time, one of the first detection capacitances CX1 decreases because the overlapping area between each fixed electrode tooth of one of the first fixed electrodes J40 and each movable electrode tooth of corresponding movable electrode portion 36 becomes smaller, while the other first detection capacitance CX2 increases because the overlapping area between each fixed electrode tooth of the other first fixed electrode J50 and each movable electrode tooth of corresponding movable electrode portion 36 becomes larger. In that case, the output S′ is expressed in eq. 3:                               S          ′                =                                            {                                                (                                      CY2                    -                    CY1                                    )                                +                                  (                                      CX2                    -                    CX1                                    )                                            }                        ×                          Vcc              /              Cf                                =                                                    [                                                      {                                                                  (                                                  C0                          +                                                      Δ                            ⁢                                                                                                                  ⁢                            Cy                                                                          )                                            -                                              (                                                  C0                          -                                                      Δ                            ⁢                                                                                                                  ⁢                            Cy                                                                          )                                                              }                                    +                                      {                                                                  (                                                  C0                          +                                                      Δ                            ⁢                                                                                                                  ⁢                            Cx                                                                          )                                            -                                              (                                                  C0                          -                                                      Δ                            ⁢                                                                                                                  ⁢                            Cx                                                                          )                                                              }                                                  ]                            ×                              Vcc                /                Cf                                      =                                                            {                                                            (                                              C0                        +                                                  Δ                          ⁢                                                                                                          ⁢                          Cy                                                -                        C0                        +                                                  Δ                          ⁢                                                                                                          ⁢                          Cy                                                                    )                                        +                                          (                                              C0                        +                                                  Δ                          ⁢                                                                                                          ⁢                          Cx                                                -                        C0                        +                                                  Δ                          ⁢                                                                                                          ⁢                          Cx                                                                    )                                                        }                                ×                                  Vcc                  /                  Cf                                            =                              2                ×                                  (                                                            Δ                      ⁢                                                                                          ⁢                      Cx                                        +                                          Δ                      ⁢                                                                                          ⁢                      Cy                                                        )                                ×                                  Vcc                  /                  Cf                                                                                        (                  eq          .                                          ⁢          3                )            
As shown in eq. 3, when detecting acceleration along the second direction Y, in addition to the expected output 2ΔCy, the differential output 2ΔCx from the first detection capacitances CX1, CX2, which are used to detect the acceleration detection in the first direction X, is also picked up as noises.
Therefore, in the proposed sensor in FIG. 8, whether the sensor is under acceleration along the first direction X or the second direction Y, capacitance changes that are not used for detecting the acceleration are included as noises in the output of the sensor.